Fluid streams comprising hydrogen sulfide and ammonia are produced in a number of industrial processes. For example, “sour water”, which comprises hydrogen sulfide and ammonia, is generated in refinery processes through various water wash processes. Sour water is stripped with steam to remove hydrogen sulfide and ammonia, generating “sour water stripper gas” (SWS gas) that typically contains about equal amounts of ammonia, hydrogen sulfide and water, although may contain up to 50% by volume ammonia in addition to water vapor, hydrogen sulfide and trace hydrocarbons.
In most refineries the SWS gas is treated in a Claus sulfur recovery unit (SRU) in which ammonia is reduced to nitrogen and hydrogen. The potentially valuable fixed nitrogen content of SWS gas is not only lost in this process, but also the operation of a SRU is negatively affected by the presence of ammonia in the feed (i.e., SWS gas). The negative effects to a SRU include increased air demand in an oxidation step, need for higher furnace temperature, reduced unit capacity and higher salt formation.
Alternatively, SWS gas may be treated in spent acid recovery (SAR) plants with conventional furnace technology. In these processes, the ammonia in SWS gas is converted to nitrogen and NOX. Thus, it is generally expected that the presence of ammonia will increase NOX generation and reduce capacity of a SAR plant.
Coke ovens also produce a fluid stream comprising hydrogen sulfide and ammonia as hot, “raw” coke oven gas. “Raw” coke oven gas may be conditioned through several steps to yield a gas that can be used as a clean fuel. “Raw” coke oven gas comprises ammonia, which, due to its corrosive nature, must be removed. Ammonia is typically removed by contacting the raw coke oven gas with sulfuric acid, yielding ammonium sulfate.
More modern processes for ammonia removal from coke oven gas include washing (scrubbing) with water into which ammonia, in addition to hydrogen sulfide and hydrogen cyanide, if present, dissolve, thus removing the contaminants from the coke oven gas. The resulting scrubbing solution is transferred to an ammonia still in which steam is used to strip the ammonia from the solution. Ammonia vapor from the still may be processed to form ammonium sulfate, i.e., by reacting with sulfuric acid. Ammonia vapor may alternatively be condensed to form a strong ammonia solution, incinerated, or catalytically converted to nitrogen and hydrogen, which are then recycled back into the coke oven gas. The incineration of the ammonia vapors is usually not an option in areas where environmental laws restrict the emission of NOX.
Kresnyak et al., in EP 0 857 509 A1, disclose a process for scrubbing ammonia and hydrogen sulfide from a fluid acid stream, including recovering the ammonia as ammonium sulfate. Kresnyak et al. disclose treating the stream with sulfuric acid to remove hydrogen sulfide from the stream and convert ammonia present in the stream to ammonium sulfate. Kresnyak et al. suggest using an optional charcoal filter to remove residual hydrogen sulfide and hydrocarbons from the ammonium sulfate solution. Rigorous removal of the hydrogen sulfide from the ammonium sulfate stream by charcoal filtration to insure absence of any hydrogen sulfide odor in the ammonium sulfate product is expensive and challenging.
It is desirable to have a process to separate ammonia from fluid streams comprising hydrogen sulfide and ammonia in a useful form and to reduce residual hydrogen sulfide levels in the separated ammonia to below the odor threshold (less than about 0.1 mg/Mg). It is also desirable to minimize NOX generation from conventional SAR plants, increase capacity and reliability of existing SR and SAR plants, and eliminate premature salt formation from ammonia, carbon dioxide and hydrogen sulfide in SR units. The present invention provides such a process.